1. Field of the Invention
The present invention relates to the cleaning of a semiconductor wafer and more specifically the cleaning of a semiconductor wafer exhibiting, after plasma etching, traces of a residual polymer especially containing aluminum.
2. Discussion of the Related Art
The manufacturing of integrated circuits from silicon wafers requires many steps of etching and implantation of various materials according to some predetermined patterns. A specific step consists of creating, on a silicon oxide surface, patterns formed of aluminum. Silicon oxide is conventionally boron and phosphorus-doped (BPSG). During this step, a uniform aluminum layer is deposited over the entire surface of the silicon oxide, then this aluminum layer is covered with a uniform resist layer on which are formed, by a conventional photolithographic etching method, the patterns that are desired to be reproduced on the aluminum.
The wafer is then submitted to a plasma etching which will etch the aluminum anisotropically in a direction substantially perpendicular to the wafer surface and form the desired patterns in the aluminum. This plasma etching operation consists of digging vertical holes of various cross-sections in the aluminum. At the end of the etching, it can be seen that a thin film especially formed of a polymer including aluminum, boron, chlorine, and carbon, has been deposited on the substantially vertical walls of the hole.
This conductive film is a source of defects and it is desired to eliminate it.
A conventional method to remove the polymer film consists of cleaning the surface of the silicon wafer with a specific solvent, available under denomination EKC.265. This solution has several disadvantages. A cleaning operation using this solvent is relatively long, which represents a significant cost. The solvent used is highly inflammable, and its handling is delicate and costly. Further, the solvent used is likely to harm the environment. It has to be recycled with great precautions, which is long and costly. It is thus desired to avoid using this solvent.
Another cleaning method consists of submitting the wafer to a corrosive atmosphere composed of hydrofluoric acid in gas form and of water vapor for a given time, then rinsing the wafer with water. A disadvantage of this method is that the corrosive atmosphere used to remove the polymer strongly etches the silicon oxide layer located under the aluminum. As a result, the height between the upper surface of the aluminum layer and the upper surface of the oxide layer increases, and the oxide layer may be pierced. Even in the absence of such a piercing, the above-mentioned height increase corresponds to a step height increase for the following layers to be formed, which can create difficulties in following manufacturing steps, for example, during the planarization of a subsequently deposited dielectric layer.